1. Field of the Invention
The present invention relates to a novel fuel cell.
2. Description of Related Art
Global warming and environmental pollution due to heavy consumption of fossil fuels have become serious more and more. As a possible solution to these issues, fuel cells, such as polymer electrolyte fuel cells that use hydrogen or methanol as a fuel, and oxygen or oxygen-containing gas such as air as an oxidizing agent, have received attention as an alternate for internal-combustion engines that operate with the burning of fossil fuels.
The fuel cells receive attention as clean power generation systems; exhausts of power generation by fuel cells give less influence on the environment. Particularly, they are expected to be used as distributed power sources and as power sources for electric automobiles. In addition, there are attempts to apply fuel cells to high-energy-density power sources for mobile devices.
The fuel cells are roughly classified into as polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells, et al. Among them, the polymer electrolyte fuel cells can generate power at relatively low temperatures ranging from room temperature to about 100° C. and an output power with high density, whereby they are most suitable for the above-mentioned uses.
The polymer electrolyte fuel cells generally essentially include a polymer electrolyte, and a pair of carbon layers disposed outside the polymer electrolyte, in which the pair of carbon layers carry catalytic metals for constituting an anode and a cathode, respectively. Hereinafter the carbon layers carrying the catalytic metals are referred as electrode catalyst layers. In addition, an anode gas diffusion layer is disposed adjacent to a side of the anode opposite to the polymer electrolyte, and a cathode gas diffusion layer is disposed adjacent to a side of the cathode opposite to the polymer electrolyte. These gas diffusion layers serve to smoothly diffuse a fuel such as hydrogen or methanol and an oxidizing agent such as air or oxygen gas, respectively.
In the polymer electrolyte fuel cell, hydrogen, if used as a fuel, in the fuel and oxygen in air react each other according to the following Formulae to yield water during power generation.Anode side: H2→2H++2e−  (1)Cathode side: ½O2+2H++2e−→H2O  (2)Total reaction: H2+½O2→H2O  (3)
This indicates that the polymer electrolyte fuel cell should smoothly carry out diffusion and migration of hydrogen and oxygen toward the catalysts and water produced as a result of the power generation reactions must be immediately discharged, because the produced water impedes the diffusion and migration of hydrogen and oxygen toward the catalysts.
Accordingly, management of material diffusion, such as feeding of gases to the electrodes and removal of produced water, is a very technical important matter in the polymer electrolyte fuel cell. Particularly, in the cathode gas diffusion layer, air is moisturized before being fed to the cathode in order to render the polymer electrolyte moisturized to there by improve proton conductivity, and, as shown in Formula (2), water is produced as a result of power generation. Accordingly, the cathode and cathode gas diffusion layer are likely to suffer from flooding in which water accumulates in the cathode and cathode gas diffusion layer and impedes the diffusion of the gas.
In the known fuel cells, a gas diffusion layer substrate made typically of a carbon paper or carbon cloth is treated to be water repellent. In the treatment, the gas diffusion layer substrate is impregnated with a dispersion of a water-repellent polymer (a dispersion of fine particles of a water-repellent polymer in an aqueous medium), and the immersed substrate is dried and fired. Examples of such water-repellent polymer include polytetrafluoroethylene (hereinafter abbreviated as PTFE) and copolymers of tetrafluoroethylene and hexafluoropropylene (FEP). Although obtaining the function of discharging water from the gas diffusion layer itself, the resulting gas diffusion layer has pores with sizes larger than the size of the electrode catalyst layer, and this makes it difficult to diffuse the produced water from the electrode catalyst layer into the gas diffusion layer and to discharge toward gas flow passages.
Patent Document 1 discloses a polymer electrolyte fuel cell that includes a gas diffusion electrode. The gas diffusion electrode is composed of an electrode catalyst layer, a porous substrate supporting the electrode catalyst layer, and a water-repellent material applied to the porous substrate, in which the electrode catalyst layer is composed of a polymer electrolyte and powdery carbon carrying catalysts, and the porous substrate is composed of a carbon material. In this gas diffusion electrode, the content of the water-repellent material in the porous substrate continuously varies from one side of the porous substrate adjacent to the electrode catalyst layer to the other side.
Patent Document 2 discloses a gas diffusion layer including a gas diffusion layer substrate and a water-repellent carbon layer, and further including a water-retaining layer between the gas diffusion layer substrate and the water-repellent carbon layer, still further having drainage performance and water retention performance. This configuration improves drainage performance while carrying out moisturization of the electrolyte membrane.
Patent Document 3 discloses a gas diffusion layer having a gas diffusion layer substrate and a water-repellent carbon layer, in which the water-repellent carbon layer includes at least two layers composed of materials having different water repellencies, and one of the at least two layers closer to an electrode has a higher water repellency than that of another layer. Patent Document 3 describes that this improves drainage performance of produced water from the electrode to the gas diffusion layer and improves moisture retention performance and gas diffusivity of electrode.
Patent Document 1: Japanese Patent Laid-open No. 2003-109604
Patent Document 2: Japanese Patent Laid-open No. 2006-79938
Patent Document 3: Japanese Patent Laid-open No. 2006-4879
The gas diffusion electrode disclosed in Patent Document 1 has water repellency that continuously varies and is thereby expected to discharge water more satisfactorily. This prior art, however, lacks a water-repellent carbon layer between the gas diffusion layer substrate and the electrode catalyst layer. Although this configuration improves the drainage performance in the gas diffusion layer, it impedes the diffusion of the produced water from the electrode catalyst layer toward the gas diffusion layer. The produced water may there by accumulate in the electrode catalyst layer.
The gas diffusion layer disclosed in Patent Document 2 includes the water-retaining layer between the gas diffusion layer substrate and the water-repellent carbon layer, whereby the drainage performance is improved while rendering the electrolyte membrane moisten. However, when operated at a high humidity and a high current density (e.g., 1 A/cm2 or more), produced water in excess accumulates in the water-retaining layer, whereby the gas diffusivity may decrease.
The gas diffusion layer disclosed in Patent Document 3 has the gas diffusion layer substrate and the water-repellent carbon layer, in which the water-repellent carbon layer includes at least two layers composed of materials having different water repellencies, and one of the at least two layers closer to the electrode has a water repellency higher than that of another layer. This improves drainage performance of produced water from the electrode to the gas diffusion layer, humidity retention performance and gas diffusivity. However, although water repellency in the water-repellent carbon layer is improved, drainage performance in the gas diffusion layer substrate according to this prior art may be sufficient, and gas diffusivity may deteriorate.
Accordingly, an object of the present invention is to provide a fuel cell whose drainage performance of produced water in the gas diffusion layer is improved, whereby the fuel cell generates power with high performance.